CI: downgrade Python lint task to Ubuntu 22.04

Ubuntu 22.10 was EOLed on 2023-07-26.¹

The obvious thing would be to _upgrade_ to Ubuntu 23.04. But we run into the new
Python policy Ubuntu has instituted (see
0050eac). We could comply with this or move
this task to Fedora, but then we find that Pylint does not work with Python
3.11² which is what is in use on both Ubuntu 23.04 and all supported Fedora
releases. So downgrading to Ubuntu 22.04 seems like the simplest options for an
environment that is still in its support window.

¹ https://wiki.ubuntu.com/Releases
² Discussed in the comments of
  pylint-dev/pylint#6535. Symptoms are complaints of
  `os` members not existing.

.gitlab-ci.yml

@@ -1353,10 +1353,10 @@ lint_html:
  - tags
 
 lint_python:
- image: "$CI_REGISTRY_IMAGE/ubuntu-22.10:$CI_COMMIT_SHA"
+ image: "$CI_REGISTRY_IMAGE/ubuntu-22.04:$CI_COMMIT_SHA"
  stage: test
  needs:
- - docker_build_ubuntu-22.10
+ - docker_build_ubuntu-22.04
  script:
  - git ls-files -z -- '**.py' | xargs -0 -- python3 -m pylint --rcfile=.pylintrc --disable=fixme
  - git ls-files -z -- '**.py' | xargs -0 -- python3 -m black --check